This application claims the priority of Swiss Application No. 1835/99 filed Oct. 7, 1999, which is incorporated herein by reference.
Multi-layer films made of synthetic material for packaging various products are well known in the art. Such films are composed of at least one carrier filmxe2x80x94which may be provided with a printed messagexe2x80x94and a hot sealing layer. By a layered combination of base materials having different properties a composite film may be obtained which has a property profile that is superior to films consisting of a single layer.
As disclosed in U.S. Pat. No. 5,811,185, for the conventional sealing processes performed with electrically heated sealing shoes, particularly the combination of a first-melting layer (sealing layer) with a mechanically and thermally stable carrier layer is of importance. The manufacture of such composite films of different polymer materials is, however, complex and expensive. In the alternative, high-frequency, induction and light sealing processes have been used. In each of these processes one part of the electromagnetic energy must be absorbed by the composite film. In the high-frequency sealing process such a result is achieved by the selection of the polymer materials having a sufficiently high dielectric loss factor, as disclosed in U.S. Pat. No. 5,840,146. Similarly, electrically conducting layers are needed for the induction sealing process, as disclosed in U.S. Pat. No. 5,635,011. Composite profiles having metallic or at least electrically conducting layers are, however, expensive to make and are not desirable in all cases of application. For the light sealing process pigments or dyes are added to the film material for reinforcing the optical absorption, as discussed in German Offenlegungsschrift (application published without examination) U.S. Pat. No. 3,813,570. The sealing process disclosed in this patent document and the films used therefor, however, are not adapted for providing transverse sealing seams on wrapper sleeves made of the film.
It is an object of the invention to provide an improved multi-layer synthetic film which makes possible an efficient sealing and is also adapted to receive transverse sealing seams thereon.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the multilayer synthetic packaging film includes an at least partially transparent carrier layer having opposite first and second faces and a light-absorbing absorption layer applied to the first face of the carrier layer and having a maximum thickness of 10 micron.
According to further advantageous features of the invention, the absorption layer is a thermoplastic material and further, on that face of the absorption layer which is oriented away from the carrier layer, a thermoplastic sealing layer is provided. The total thickness of the absorption layer and the sealing layer is 10 micron at the most. Such a construction makes possible a particularly efficient sealing of two films with one another. Since only a total layer thickness of maximum 2xc3x9710 micron is heated, such a heating occurs very rapidly. Since, during this occurrence the two carrier layers remain cool, the tension strength weakens only slightly during the sealing process. Further, an efficient heat removal occurs in these layers subsequent to the sealing process so that the sealing seam solidifies very rapidly and thus the full strength of the seam is rapidly reached. Also, extraordinarily short sealing times are obtained. Because the carrier layer remains cool, it does not need to have a higher melting temperature than the sealing layer and it does not adhere to the sealing shoes or guide elements which press the sealing seam during the sealing process. For this reason the composite film may be made in an inexpensive manner from one and the same material, except that the absorption layer should be pigmented or dyed. The absorption layer is preferably at least 50% transparent, that is, the degree of absorption is 50% at the most. As a result, in case of two superposed films, whose sealing layers are oriented toward one another, both sealing layers are simultaneously heated by means of light absorption in the course of an only unilateral irradiation by the light source. In case a degree of absorption of less than 25% is selected, simultaneously two or more superposed film pairs may be sealed with one and the same light pulse. Such a proceeding is advantageous, for example, in case of pinch-folds or folded-over edge zones. The film is adapted to be provided with arbitrarily arranged sealing seams. Thus, it is adapted, for example, to be transversely sealed when in a wrapper sleeve form to function as packaging material.
According to a preferred embodiment of the invention, the film has the following layers: a first transparent layer, a second transparent layer and a dyed or pigmented absorption layer wherein the thickness ratio of the three layers are approximately 4:2:1 and wherein preferably all three layers are made of the same basic material, preferably polyolefin. This construction lends itself particularly well for a co-extrusion process: it has been found that it is relatively difficult to simultaneously extrude a relatively thick carrier layer together with a very thin absorption layer or sealing layer to obtain a conventional composite film.